In recent years, with the improvement of environmental awareness, lead-free solder has been widely used in different fields. An Sn—Ag—Cu (SAC) alloy is the most commonly used alloy for producing lead-free solder. In addition, there are other two types of alloy compositions at present. One is an Sn—Ag—Cu—Ni (SACN) alloy formed by adding nickel (Ni) into the SAC alloy. When the SACN alloy is formed into solder, the addition of Ni element could suppress copper element consumption of a copper pad and slow down the fast growth of intermetallic compounds (IMC). The other is an Sn—Ag—Cu—Ni—Bi (SACNB) alloy formed by adding nickel (Ni) and bismuth (Bi) into the SAC alloy. When the SACNB alloy is formed into solder, mechanical properties such as excellent and better hardness, yield strength and tensile strength could be achieved, and in a dense and precise packaging process, solder joints of the solder also have better performance in a thermal cycling test (TCT).
In general, after the solder formed by the above SACN and SACNB alloys is subjected to reflows for multiple times and tested by a solder ball shear test according to the automotive specification of AEC-Q100, when the ball shear speed is above 400 μm/s, the interface strength of the solder joints is often reduced due to the multiple reflows of the solder joints, and thus brittle fracture of an intermetallic compound layer is likely to occur, or failure tends to happen in the process of thermal cycling test at level 1 (−50 □ to +150 □, 1000 cycles), which results in that the soldering strength could not reach the standards required by AEC-Q100.